Recently, in line with the development of information and telecommunications industry, electronic devices are being miniaturized, light-weighted, reduced in thickness, and portable. As a result, the need for high energy density batteries used as power sources of such electronic devices has increased. Currently, research into lithium secondary batteries, as batteries that may best satisfy the above need, has actively conducted.
A lithium secondary battery is a battery which is composed of a positive electrode, a negative electrode, and an electrolyte and a separator which provide movement paths of lithium ions between the positive electrode and the negative electrode, wherein electrical energy is generated by oxidation and reduction reactions that occur when lithium ions are stored in and released from the positive electrode and the negative electrode.
A lithium secondary battery has an average discharge voltage of about 3.6 V to about 3.7 V, and one of the advantages of the lithium secondary battery is that it has a higher discharge voltage than other alkaline batteries and a nickel-cadmium battery. In order to achieve such a high operating voltage, an electrolyte composition, which is electrochemically stable in a charge and discharge voltage range of 0 V to 4.2 V, is required.
Lithium ions released from a positive electrode active material, such as lithium metal oxide, during initial charging of a lithium secondary battery move to a negative electrode active material, such as a graphite-based material, to be intercalated into interlayers of the negative electrode active material. In this case, since lithium is highly reactive, lithium reacts with an electrolyte and carbon constituting the negative electrode active material on a surface of the negative electrode active material, such as a graphite-based material, to form a compound such as Li2CO3, Li2O, or LiOH. These compounds may form a solid electrolyte interface (SEI) film on the surface of the negative electrode active material such as a graphite-based material.
The SEI film may only pass lithium ions by acting as an ion tunnel. Due to the effect of the ion tunnel, the SEI film may prevent the destruction of a negative electrode structure due to the intercalation of organic solvent molecules having a high molecular weight, which move with lithium ions in the electrolyte, into the interlayers of the negative electrode active material. Thus, it has been reported that the decomposition of the electrolyte does not occur by preventing the contact between the electrolyte and the negative electrode active material, and stable charge and discharge may be maintained by reversibly maintaining the amount of lithium ions in the electrolyte.
However, the SEI film of the lithium secondary battery may be unstable due to an additive or an organic solvent included in the electrolyte, and although the SEI film may be stably formed, gas may be generated due to the decomposition of the residual additive.
Even in a case of using an imide-based salt which may improve high-temperature storability and low-temperature output characteristics by minimizing an increase in viscosity of the organic solvent at low temperature and improving the mobility of the lithium ions, there is a significant limitation in using the lithium secondary battery due to the possibility of corrosion.
Thus, there is an urgent need to develop a method of improving output characteristics and lifetime characteristics of the lithium secondary battery while forming a robust and uniform SEI film and selectively selecting and using an electrolyte.